1. Field of the Invention
The invention generally relates to a patterning system, and more particularly, to a method for designing a patterning system.
2. Description of Related Art
In recent years, along with the micronization of semiconductor devices, micro machining techniques for depicting circuit patterns by using charged particle rays (for example, electron beam and ion beam) have been developed in order to break the resolution limit of optical systems.
It has become very difficult to form fine patterns through ultraviolet lithography along with the micronization of semiconductor devices. Thus, manufacturers in the industry have proposed and started to develop resolutions based on lithography techniques using X-rays, electron beams, and ion beams, etc. Previously proposed electron beam printing lithography techniques include a projection exposure with variable axis immersion lenses (PREVAIL), a scattering with angular limitation in projection electron-beam lithography (SCALPEL), and a low energy electron-beam proximity projection lithography (LEEPL).
In the PREVAIL and the SCALPEL techniques, a high-energy electron beam with an acceleration voltage of about 100 kV is passed through part of a mask and projected onto a photoresistor in a quarter of its original size to produce and print a pattern. In the LEEPL technique, a low-energy electron beam with an acceleration voltage of about 2 kV is passed through a cavity formed on a mask to print a pattern on a photoresistor in an equal size. Generally speaking, the higher an electron acceleration voltage is, the less dispersed the electrons in a photoresistor are and accordingly the less chance the electrons have to react with the photoresistor. Thus, a highly sensitive photoresistor has to be adopted in a lithography technique using a high-energy electron beam. Contrarily, because the LEEPL technique adopts a low-energy electron beam, it offers a high photoresistive sensitivity, a high resolution, and a low substrate damage rate.
During a patterning process in a conventional lithography technique, the focusing properties (for example, the size of the focal point, the depth of focus, the distance between an electron-optical system and the focal point, etc.) of the electron beam are usually adjusted according to the specification of the pattern. However, there may be more factors in the formation of a pattern. Thus, the patterning result may not be satisfactory if an electron-optical system is designed by taking only the focusing properties of an electron beam into consideration.